This invention relates generally to the field of waveform equalization, and more specifically to the field of ghost cancelers for television, and more specifically to a storage-mode acoustic charge transport device used as a ghost canceller for television.
Ghosts in television images are caused by multi-path transmission channels. In general, they are time-delayed, attenuated, and distorted versions of the intended signal. They also can be time-varying when caused by reflections from moving objects, for example airplane flutter. In conventional NTSC or analog HDTV they are an irritating nuisance which can seriously impair picture quality and visibility. In the case of terrestrial broadcast of analog HDTV signals, even small amounts of multipath can reduce the picture quality to a level which is no better than present day NTSC.
In digital HDTV broadcasts the situation is even more serious, since digital schemes do not degrade gracefully under signal impairment as do analog transmission systems. This is especially true of the digital HDTV broadcast systems which utilize bandwidth compression techniques such as Huffman coding and Run-Length Limiting. Such techniques often transmit symbols of varying lengths which represent various signal levels. Signal levels which repeat often are represented by symbols which are only a few bits long, whereas signal levels which occur rarely are represented by longer symbols. In order to decipher a stream of variable length symbols, the receiver decoder must not make any mistake or it can become completely lost.
The ghost problem is forcing HDTV proponents to develop adaptive equalizers, or ghost cancelers, for their systems. They are developing digital equalizers because no satisfactory analog equalizer technology has existed until now. The basic function of an equalizer, to remove the effects of multipath and perhaps other distortions, can be understood in either the time or frequency domain. In the time domain, it may be shown that the equalizer cancels out the distortion components from the received signal by creating a copy of just the distortion components, and then subtracting this from the incoming signal. In the frequency domain, the undesired signal components cause the frequency response of the transmission channel to be distorted, usually having periodic ripples in the channel's frequency response. The function of an equalizer may be viewed in the frequency domain as an inverse filter, having ripples in its frequency response which are exactly opposite to those of the distorted transmission channel. When this equalizer is used in the receiver, the product of the channel's frequency response and that of the equalizer will be flat. The time domain and frequency domain explanations of an equalizer are entirely equivalent.
A key component of an equalizer is a Programmable Transversal Filter or PTF. Required characteristics of a PTF include the following:
its bandwidth must be wide enough to handle the signals of interest; PA1 it must contain enough independently programmable taps, spaced close enough to create an accurate replica of the distortion (ghost) signal; PA1 its tap weight magnitudes must have sufficient resolution to create an accurate replica of the ghost signals; and PA1 it must be sufficiently long to recreate the expected delays of the distortion signals.
There has been an appreciable effort over the past decade to develop ghost cancelers for NTSC television. Most of the approaches have been done at baseband, after the video detector. Baseband operation in itself creates problems. Envelope detection cannot be used, because it freezes into the detected signal artifacts which are caused by interaction between the desired signal and the ghost. These artifacts cannot be removed subsequently by a linear operation. Synchronous detection must therefore be used if baseband ghost cancellation is to be employed. However, carrier phase recovery is disturbed by the presence of ghosts. A narrowband synchronous detector will detect on the average phase of the received signal, which depends on the relative amplitude and carrier phase of any ghost signals present. As a result, quadrature components will appear at the output of the synchronous detector for all but a few trivial cases. Multiple transversal filters must be used to handle in-phase and quadrature components. In addition, cross terms in ghosted baseband video require four transversal filters in a lattice configuration to completely cancel the interfering signals.
The BTA (Japanese equivalent of the U.S. National Association of Broadcasters) has proposed an NTSC ghost canceler based on baseband digital signal processing technology. However, digital transversal equalizers are expensive and slow. A digitally programmable analog transversal equalizer technology is needed, with satisfactory performance and affordable cost.